The lower limit of particle sizing in a gas (including air) by scattering due to illumination (such as provided by a laser source) is limited by noise generated by the background light produced by scattering from gaseous molecules. In particular, when the volume illuminated is large, as is required, for example, for high sample volume flow, when pressures are high raising molecular density, or when the intensity of illumination is itself sufficiently high, the scattering by molecules within the "viewing volume" can exceed that of the smallest particle attempting to be sized by manyfold.
In a similar manner, the intrinsic scattering by liquids limits the ability to measure extremely small particles therein. In addition, with respect to surfaces, the scattering from surface roughness, texture, or morphological material state similarly limits how small contaminants, or contaminating particles, can be sized or point defects measured.
To develop practical aerosol instruments capable of detecting submicron particles with high sensitivity in the presence of a high molecular scattering environment, it has been found necessary to employ multi-element detectors in combination with imaging systems of high light gathering power to divide the illuminated volume a sufficient number of times so that the molecular scattering background and its noise do not mask the scattering by the particles of interest (see my U.S. Pat. Nos. 4,798,465 and 4,893,928 wherein the use of gas as the fluid, multi-element linear arrays having typically ten to twenty elements covering a 1.times.10 .mu.m field, as the detector, and electronics for processing enable the device described to have a particle sizing sensitivity at least as low as 0.1 .mu.m diameter at a flow rate of 1 cfm using a 1 watt laser cavity, and also see my pending U.S. patent Application Ser. No. 07/919,983, and application is a continuation of U.S. patent application Ser. No. 07/505,831 now abandoned, and describes a device wherein liquid is utilized as the fluid).
In general, the higher the pixel density (i.e., number of elements utilized), the greater the noise reduction.